In Eukaroytes, gene expression is affected, in part, by the stability and structure of the messenger RNA (mRNA) molecule, mRNA stability influences gene expression by affecting the steady-state level of the mRNA; it can affect the rates at which the mRNA disappears following transcriptional repression and accumulates following transcriptional induction. The structure and nucleotide sequence of the mRNA molecule can also influence the efficiency with which these individual mRNA molecules are translated.
The intrinsic stability of a given mRNA molecule is influenced by a number of specific internal sequence elements which can exert a destabilizing effect on the mRNA. These elements may be located in any region of the transcript, and e.g., can be found in the 5′ untranslated region (5′UTR), in the coding region and in the 3′ untranslated region (3′UTR). It is well established that shortening of the poly(A) tail initiates mRNA decay (Ross, Trends in Genetics, 12:171-175, 1996). The poly(A) tract influences cytoplasmic mRNA stability by protecting mRNA from rapid degradation. Adenosine and uridine rich elements (AUREs) in the 3′UTR are also associated with unstable mammalian mRNA's. It has been demonstrated that proteins that bind to AURE, AURE-binding proteins (AUBPs), can affect mRNA stability. The coding region can also alter the half-life of many RNAs. For example, the coding region can interact with proteins that protect it from endonucleolytic attack. Furthermore, the efficiency with which individual mRNA molecules are translated has a strong influence on the stability of the mRNA molecule (Herrick et al., Mol Cell Biol. 10, 2269-2284, 1990, and Hoekema et al., Mol Cell Biol. 7, 2914-2924, 1987).
The single-stranded nature of mRNA allows it to adopt secondary and tertiary structure in a sequence-dependent manner through complementary base-pairing. Examples of such structures include RNA hairpins, stem loops and more complex structures such as bifurcations, pseudoknots and triple-helices. These structures influence both mRNA stability, e.g., the stem loop elements in the 3′UTR can serve as a endonuclease cleavage site, and affect translational efficiency.
In addition to the structure of the mRNA, the nucleotide content of the mRNA can also play a role in the efficiency with which the mRNA is translated. For example, mRNA with a high GC content at the 5′untranslated region (UTR) may be translated with low efficiency and a reduced translational effect can reduce message stability. Thus, altering the sequence of a mRNA molecule can ultimately influence mRNA transcript stability, by influencing the translational stability of the message.
Factor VIII and Factor IX are important plasma proteins that participate in the intrinsic pathway of blood coagulation. Their dysfunction or absence in individuals can result in blood coagulation disorders, e.g., a deficiency of Factor VIII or Factor IX results in Hemophilia A or B, respectively. Isolating Factor VIII or Factor IX from blood is difficult, e.g., the isolation of Factor VIII is characterized by low yields, and also has the associated danger of being contaminated with infectious agents such as Hepatitis B virus, Hepatitis C virus or HIV. Recombinant DNA technology provides an alternative method for producing biologically active Factor VIII or Factor IX. While these methods have had some success, improving the yield of Factor VIII or Factor IX is still a challenge.
An approach to increasing protein yield using recombinant DNA technology is to modify the coding sequence of a protein of interest, e.g., Factor VIII or Factor IX, without altering the amino acid sequence of the gene product. This approach involves altering, for example, the native Factor VIII or Factor IX gene sequence such that codons which are not so frequently used in mammalian cells are replaced with codons which are overrepresented in highly expressed mammalian genes. Seed et al., (WO 98/12207) used this approach with a measure of success. They found that substituting the rare mammalian codons with those frequently used in mammalian cells results in a four fold increase in Factor VIII production from mammalian cells.